1. I marginally prefer
Keavy, which is sad since there isn't really an Irish spelling equivalent for that.
Caoimhe is really really great though, I especially love the spelling.
2. I don't think it needs an Irish mn, but some would work.
3.
Caoimhe Tamsin Caoimhe Bethany Caoimhe Josephine Caoimhe Emmeline Caoimhe Sinead